Electrolysis cell and cathode with irregular surface profiling

ABSTRACT

An electrolysis cell for the production of aluminum has a liquid aluminum layer on a cathode, a melt layer on the liquid aluminum, and an anode above the melt layer. An upper side of the cathode has surface profiling with two or more elevations provided at at least two of the twenty points of the surface of the upper side of the cathode vertically beneath those regions of the boundary surface between the layer of liquid aluminum and the melt layer in which peaks with the twenty highest maxima are present in the distribution of a reference wave formation potential in the boundary surface. The reference wave formation potential is defined as the wave formation potential which, when the electrolysis cell is operated with a reference cathode without surface profiling is present at a point in the boundary surface between the layer of liquid aluminum and the melt layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation, under 35 U.S.C. §120, of copendinginternational patent application No. PCT/EP2012/057524, filed Apr. 25,2012, which designated the United States; the application also claimsthe priority, under 35U.S.C. §119, of German patent application No. 102011 076 302.3, filed May 23, 2011; the prior applications are herewithincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an electrolysis cell, in particular forthe production of aluminum, as well as a cathode which is suitable foruse in such an electrolysis cell.

Electrolysis cells are used for example for the electrolytic productionof aluminum, which is carried out industrially usually according to theHall-Héroult process. In the Hall-Héroult process, a melt composed ofaluminum oxide and cryolite is electrolyzed. The cryolite, Na₃[AlF₆], isused to lower the melting point of 2,045° C. for pure aluminum oxide toapprox. 950° C. for a mixture containing cryolite, aluminum oxide andadditives, such as aluminum fluoride and calcium fluoride.

The electrolysis cell used in this process comprises a cathode base,which can comprise a large number of cathode blocks lying adjacent toone another forming the cathode. In order to withstand the thermal andchemical conditions prevailing during operation of the cell, the cathodeis usually composed of a carbon-containing material. Grooves are usuallyprovided in each case at the undersides of the cathode, in which groovesthere is disposed in each case at least one busbar through which thecurrent fed via the anodes is carried away. Disposed approx. 3 to 5 cmabove the usually 15 to 50 cm high layer of liquid aluminum present onthe upper side of the cathode is an anode constituted by individualanode blocks, the electrolyte, i.e. the melt containing aluminum oxideand cryolite, being located between the latter and the surface of thealuminum. During the electrolysis carried out at approx. 1,000° C., theformed aluminum is deposited beneath the electrolyte layer on account ofits greater density compared to that of the electrolyte, i.e. as anintermediate layer between the upper side of the cathode and theelectrolyte layer. During the electrolysis, the aluminum oxide dissolvedin the melt is split up by the electric current flow to form aluminumand oxygen. Viewed electrochemically, the layer of liquid aluminum isthe actual cathode, since aluminum ions are reduced to elementaryaluminum at its surface. Nonetheless, the term cathode will beunderstood in the following not to mean the cathode from theelectrochemical standpoint, i.e. be layer of liquid aluminum, but ratherthe component forming the electrolysis cell base, for example comprisingone or more cathode blocks.

A significant drawback of the Hall-Héroult process is that it is veryenergy-intensive. In order to produce 1 kg of aluminum, approx. 12 to 15kWh of electrical energy is required, which accounts for up to 40% ofthe production costs. In order to be able to reduce the productioncosts, it is therefore desirable to reduce the specific energyconsumption with this process as far as possible.

On account of the relatively high electrical resistance of the melt,particularly compared to the layer of liquid aluminum and the cathodematerial, relatively high ohmic losses in the form of Joule dissipationoccur especially in the melt. In view of the comparatively high specificlosses in the melt, it is endeavored to reduce as far as possible thethickness of the melt layer and thus the distance between the anode andthe layer of liquid aluminum. However, on account of the electromagneticinteractions present during the electrolysis and the wave formation thusproduced in the layer of liquid aluminum when there is an excessivelysmall thickness of the melt layer, there is the risk of the layer ofliquid aluminum coming into contact with the anode, which can lead toshort-circuits of the electrolysis cell and to undesired reoxidation ofthe formed aluminum. Such short-circuits also lead to increased wear andthus to a reduced service life of the electrolysis cell. For thesereasons, the distance between the anode and the layer of liquid aluminumcannot be reduced arbitrarily.

In order to reduce further the specific energy consumption, electrolysiscells with cathodes have also recently been proposed, the upper side ofwhich cathodes facing the liquid aluminum and the melt during theoperation of the electrolysis cell having a surface profiling. Publishedpatent application US 2011/0056826 A1 discloses for example a cathodewith a regularly constituted surface profiling. The horizontal andvertical fluctuations in the layer of liquid aluminum are intended to bereduced by the regularly constituted surface profiling, as a result ofwhich the stability of the layer of liquid aluminum is intended to beincreased. With such a regularly constituted surface profiling, however,the wave formation in the layer of liquid aluminum is reduced only to alimited extent and in particular not uniformly over the whole cathodesurface. Furthermore, this known regular surface profiling in thecathode block surface leads, due to the reduced movement in the layer ofliquid aluminum, indirectly to a considerable hindrance of the mixing inthe melt layer located above the latter which is required for thedissolution of the periodically supplied aluminum oxide, and this provesto have a disadvantageous effect on the achievable energy efficiency ofthe electrolysis.

European patent EP 0 938 598 B1 and German patent DE 101 64 008 C1describe electrolysis cells with cathodes, which are adapted with regardto their electrical contacting from the exterior and with regard totheir specific electrical material resistance in such a way that adistribution of the electric current density as homogeneous as possiblearises at the upper side of the cathode. In the case of theseelectrolysis cells, however, a comparatively marked wave formation alsotakes place in the layer of liquid aluminum, for which reason areduction in the specific energy consumption in the electrolysis celland an increase in its service life are not possible.

BRIEF SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a electrolysiscell and cathode which overcome the above-mentioned disadvantages of theheretofore-known devices and methods of this general type and whichprovides for an electrolysis cell which, during its operation, has areduced specific energy consumption and an increased service life. Inparticular, an electrolysis cell is to be made available, in which thethickness of the melt layer is reduced without instabilities, such asshort-circuits or reoxidation of the formed aluminum, occurring in thelayer of liquid aluminum as a result of a wave formation tendency thatis thereby increased. At the same time, the electrolysis cell accordingto the invention should ensure sufficient mixing in the melt layerduring its operation.

With the foregoing and other objects in view there is provided, inaccordance with the invention, an electrolysis cell for the productionof aluminum, comprising:

a cathode having an upper side formed with a surface profiling of two ormore elevations;

during an operation of the electrolysis cell, a layer of liquid aluminumon said upper side of said cathode, a melt layer on the layer of liquidaluminum, and an anode above the melt layer;

said surface profiling of said cathode being configured and disposed insuch a way that an elevation is in each case provided at two or more oftwenty points of a surface of said upper side of said cathode that arein each case disposed vertically beneath those regions of a boundarysurface between the layer of liquid aluminum and the melt layer in whichpeaks with the twenty highest maximum values are present in adistribution of a reference wave formation potential present in theboundary surface;

the reference wave formation potential being defined as a wave formationpotential which, during the operation of the electrolysis cell with areference cathode having no surface profiling being used instead of saidcathode with said surface profiling, but having an otherwise identicalconfiguration as said cathode with said surface profiling, is present ata point in the boundary surface between the layer of liquid aluminum andthe melt layer.

In other words, the objects are achieved by making available anelectrolysis cell as claimed, and particular an electrolysis cell forthe production of aluminum which comprising a cathode, on the upper sideof the cathode a layer of liquid aluminum, thereon a melt layer whichcontains aluminum oxide and cryolite, and above the melt layer an anode,wherein the cathode comprises at its upper side a surface profilingformed by two or more elevations, wherein the surface profiling of thecathode is constituted and disposed in such a way that an elevation isin each case provided at at least two of the twenty points of thesurface of the upper side of the cathode which in each case are disposedvertically beneath those regions of the boundary surface between thelayer of liquid aluminum and the melt layer in which the peaks with thetwenty highest maximum values are present in the distribution of thereference wave formation potential present in the boundary surface,wherein a reference wave formation potential is defined as the waveformation potential which, during the operation of the electrolysis cellwith—instead of the cathode with the surface profiling—a referencecathode without surface profiling, but an otherwise identicalconfiguration to the cathode with surface profiling, is present at apoint in the boundary surface between the layer of liquid aluminum andthe melt layer.

According to the invention, the cathode of the electrolysis cellcomprises a surface profiling which, particularly with regard to theposition, the size and the shape of the individual components of thesurface profiling, is adapted in a targeted manner such that, during theoperation of the electrolysis cell, the formation of marked peaks in thewave formation potential is avoided in a targeted manner in the boundarysurface between the layer of liquid aluminum and the melt layer and,consequently, a uniform and low distribution of the wave formationpotential results, as viewed over this boundary surface, than would bethe case with the use of a corresponding cathode without surfaceprofiling.

In the sense of the present invention, surface profiling is understoodto mean the sum of all the elevations provided on the base plane of thecathode. The term base plane denotes the horizontal plane of the cathodewhich lies farthest in the direction of the anode and which runs throughthe whole cross-sectional area of the cathode, without intersecting thesurface-profiled upper side of the cathode. All the elevations providedon this base plane are therefore orientated in the direction towards theanode and are surrounded by the layer of liquid aluminum. The height ofan elevation of the surface profiling is therefore the distance of theuppermost point of the elevation from the point of the base plane of thecathode lying vertically thereunder.

In this solution according to the invention, account is taken of thefact that the wave formation potential, as defined below, in theboundary surface between the layer of liquid aluminum and the melt layerduring the operation of the electrolysis cell is the driving force forthe wave formation in the layer of liquid aluminum, and also inparticular that distribution of the wave formation potential in the caseof conventional electrolysis cells is not uniform over the boundarysurface between the layer of liquid aluminum and the melt layer, but onthe contrary is extremely heterogeneous. Due to the reduction in thewave formation potential provided according to the invention andparticularly as a result of the distribution of the wave formationpotential in the boundary surface between the layer of liquid aluminumand the melt being made uniform, a wave formation in the layer of liquidaluminum is reliably prevented or at least considerably reduced duringthe operation of the electrolysis cell according to the invention, as aresult of which the thickness of the melt layer can be reduced comparedto conventional electrolysis cells and the efficiency of theelectrolysis cell according to the invention is thus considerablyincreased.

A further important finding of the present invention is that theheterogeneous distribution of the wave formation potential present inthe boundary surface between the layer of liquid aluminum and the meltlayer in the case of conventional electrolysis cells can be directlyinfluenced by the provision and the specific configuration of thesurface profile at the upper side of the cathode of the electrolysiscell and marked peaks of the wave formation potential at individualpoints of the boundary surface can in this way be avoided in a targetedmanner. As explained below in detail, the wave formation potential at aspecific point in the aforementioned boundary surface depends on thevectorial product of the electric current density and the magnetic fluxdensity present at this point. If a specific current path is considered,which leads from the current supply of the cathode to the anode of theelectrolysis cell, the total electrical resistance along this path andconsequently the current density at the point at which the path crossesthe boundary surface between the layer of liquid aluminum and the meltlayer depends in particular on what path length of the path runsrespectively in the cathode block, in the layer of liquid aluminum andin the melt layer. Since these materials each have different specificelectrical resistance values, the melt layer and also the cathodematerial in particular having a higher specific electrical resistancethan the liquid aluminum, and because the individual current paths havedifferent path lengths in the cathode block, in the layer of liquidaluminum and in the melt layer, the total electrical resistances alongthe individual paths and thus also the individual current densities overthe boundary surface between the layer of liquid aluminum and the meltlayer are heterogeneous in conventional electrolysis cells, so thatindividual points of the boundary surface exhibit marked current densitypeaks. Through the provision and suitable adaptation of the position,the shape and the length of the elevations of the surface profiling ofthe cathode, the path lengths of the individual current paths in thevarious sections, i.e. cathode block, layer of liquid aluminum and meltlayer, are adjusted according to the present invention in such a waythat, in the region of the boundary surface, a current densitydistribution is established which is adapted such that, in the boundarysurface between the layer of liquid aluminum and the melt layer duringoperation of the electrolysis cell, no marked peaks arise in thedistribution of the wave formation potential present in this boundarysurface, as a result of which an essentially uniform and lowdistribution of the wave formation potential is guaranteed.

In order to optimize the position, the shape and the length of theelevations of the surface profiling of the cathode, the presentinvention proceeds from the distribution of the reference wave formationpotential which results during the operation of the electrolysis cellwith a conventional, unprofiled reference cathode, and provideselevations in a targeted manner at the points of the cathode surfacewhich are disposed vertically beneath the points of the boundary surfaceat which marked peaks in the distribution of the reference waveformation potential are present. During the operation of theelectrolysis cell with the surface-profiled cathode, the electriccurrent density is reduced in these regions and the wave formationpotential is thus reduced in these regions.

As explained, the reference wave formation potential is the waveformation potential which results during the operation of theelectrolysis cell with—instead of the cathode with surface profiling—areference cathode without surface profiling, i.e. with a horizontalcathode surface, but with an otherwise identical configuration to thecathode with the surface profiling. According to the embodimentspecified in claim 1, the reference electrolysis cell used to determinethe reference wave formation potential is identical to the electrolysiscell according to the invention, except for the fact that, instead ofthe surface-profiled cathode, use is made of a reference cathode inwhich the surface profiling is not provided, in which the additionalvolume on the upper side of the cathode arising due to the omission ofthe surface profiling is filled with liquid aluminum or melt—dependingon the layer in which the corresponding material is present with thesurface-profiled cathode.

Especially in cases where many elevations occupying a considerablevolume are provided on the upper side of the cathode, it is proposed inan alternative embodiment of the present invention specified in claim 2to use a reference cathode without surface profiling to determine thereference wave formation potential and to adjust the height of thisreference cathode in the electrolysis cell in such a way that, betweenthe upper side of the cathode and the anode, the same bath volume ispresent for the layers of liquid aluminum and melt as in the case of theelectrolysis cell with the surface-profiled cathode. Since, in thiscase, the reference wave formation potential relates to a referenceelectrolysis cell with the same bath volume as that of the electrolysiscell according to the invention, the reference wave formation potentialthus determined is more meaningful than that determined according toclaim 1, if the volume of the elevations of the surface profiling of thecathode accounts for at least 10%, preferably at least 20% andparticularly preferably at least 30% of the volume of the cathode.

The wave formation potential and thus the distribution of the waveformation potential can be determined by computer-supported electrical,magnetic and magneto-hydrodynamic simulation of the movement and waveformation in the layer of liquid aluminum and the melt of the respectiveelectrolysis cell.

According to the present invention, the wave formation potential at anarbitrary point of the boundary surface between the layer of liquidaluminum and the melt is defined as the absolute value of the componentof the flow rate present in the boundary surface of the melt, saidcomponent present at this point being directed in the normal directionto the boundary surface, i.e. wave formation potential=|{right arrowover (u)}·{right arrow over (n)}|, wherein {right arrow over (u)} is theflow rate of the melt as a vector and {right arrow over (n)} is thenormal vector. The boundary surface is also assumed to be permeable, sothat the wave formation potential represents a local measure of thewave-driving flow directed towards the boundary surface. In this case,the flow of the melt cannot of course be determined experimentally, forwhich reason the wave formation potential is preferably determined bythe simulation method described below.

In order to calculate the flow conditions, the electric and magneticfields are first calculated by means of simulation according to a finiteelement method (FEM) and the resulting fields are then used in thecalculation of the flow conditions, which also takes place by means ofsimulation according to a finite element method (FEM). The softwareComsol Multiphysics in the version 3.5a is used for both simulations.The boundary surface is assumed to be permeable, wherein the waveformation potential represents a local measure of the wave-driving flowdirected towards the boundary surface. The simulated electrolysis cell,which comprises busbars, the current supplies of the electrolysis cellincluding a magnetic compensation geometry if applicable, the cathode,the layer of liquid aluminum, the melt layer, the anode, if applicablean anode tree connecting the anodes and air as a surrounding medium, issplit up geometrically component by component into finite volumeelements. Insofar as the cell to the simulated, taking account of theaforementioned components, exhibits one or more planes of symmetry, onlythe part of the electrolysis cell located on one side of each plane ofsymmetry is simulated in each case and the symmetry conditions are takeninto account by corresponding boundary conditions, as will be explainedin greater detail below.

The simulation proceeds in a simplifying manner from stationaryconditions in the electrolysis cell, so that the simulation is based onthe respective stationary physical equations. Furthermore, an isothermalelectrolysis cell is assumed which is at operating temperature (970°C.).

The simulation is based on the following variables and parameters:

-   -   V: electric voltage, scalar    -   σ: electrical conductivity, scalar    -   E (bold type): electric field, vector    -   A (bold type): electric vector potential, vector    -   A_(x), A_(y), A_(z): vector potential, component    -   H (bold type): magnetic field, vector    -   J, j (bold type), {right arrow over (j)}: electric current        density, vector    -   B (bold type), {right arrow over (B)}: magnetic flux density,        vector    -   I (bold type): unit matrix, tensor    -   F (bold type): force density (sum of Lorentz force density and        gravitational force density), vector    -   u (bold type), {right arrow over (u)}: flow rate, vector    -   u (normal type), u_(x), u_(y), u_(z): flow rate, component    -   p: pressure, scalar    -   μ: viscosity, scalar    -   ρ: density, scalar    -   L_(c): characteristic length, e.g. depth of aluminum bath    -   v_(c): characteristic speed

Additional variables with turbulent flows:

-   -   μ_(T): turbulent viscosity, scalar    -   k: turbulent kinetic energy    -   ep, ε: dissipation of turbulent kinetic energy    -   lw: distance from solid boundary surfaces    -   L_(Ref): reference length scale, corresponds to characteristic        length L_(C)    -   G: reciprocal distance from solid boundary surfaces    -   P_(K): source term of turbulent kinetic energy    -   f_(u): attenuation function viscosity    -   f_(ε): attenuation function dissipation    -   R_(t): turbulent Reynold's number    -   l*: limited mixing length    -   u_(ε): turbulent dissipation rate of all grid cells    -   n (bold type), {right arrow over (n)}: normal vector to the        boundary surface between the layer of liquid aluminum and the        melt layer, vector    -   t (bold type), {right arrow over (t)}: tangential vector,        vector)    -   {right arrow over (e_(x))}, {right arrow over (e_(y))}, {right        arrow over (e_(z))}: unit vectors, Cartesian coordinate system

The constructed grids are sufficiently finely dimensioned, so thatartifacts of the grid are no longer visible when the wave potential isevaluated. These include, for example, marked peaks or conspicuouschanges along the edges of the grid. Moreover, the dependence of thesimulated values on adjusted grid fineness and slow and limitedconvergence of the simulations indicate insufficient grid fineness inthe relevant areas.

Furthermore, a quality factor of at least 0.15 is required as a qualityfactor for the overall grid when the grid is constructed, whereinquality factor q is defined according to the Manual of ComsolMultiphysics Software as follows:

TABLE 1$q = \frac{72\sqrt{3}V}{\left( {h_{1}^{2} + h_{2}^{2} + h_{3}^{2} + h_{4}^{2} + h_{5}^{2} + h_{6}^{2}} \right)^{3/2^{\prime}}}$for tetrahedral grid cells$q = \frac{36\sqrt{3}V}{\left( {\sum\limits_{i = 1}^{9}h_{i}^{2}} \right)^{3/2^{\prime}}}$for prismatic grid cells with V = volume of the grid cell and h_(i) =edge lengths of the grid cell.

In detail, the construction of the grid takes place as follows:

The air surrounding the electrolysis cell is modeled with an unlimitedsize of the grid cells, which can vary between fine regions (e.g. at themelt layer) and coarse regions (e.g. surrounding edges of the overallarrangement). The magnification factor between two adjacent grid cellsis limited to 1.65 in order to avoid distorted grid elements.

The current supplies and discharges are reproduced with grid cells withan edge length in the region of approx. 30 cm.

The layer of liquid aluminum and the melt layer are modeled such thatthe grid cells that form the boundary surface between the layer ofliquid aluminum and the melt each have an edge length in the region ofapprox. 3 cm. The melt layer is modeled such that the mean extension ofa grid cell in the vertical direction corresponds at most to half thethickness of the melt layer.

In the context of the simulation, it is assumed that the boundarysurface between the layer of liquid aluminum and the melt is not curvedand therefore runs horizontally. Accordingly, normal vector n is adoptedas vertical unit vector e_(z) and the wave formation potential isaccordingly defined as the absolute value of vertical component u_(z) ofthe flow rate in the boundary surface.

The layer of liquid aluminum and the cathode are modeled such that thegrid cells that form the boundary surface between the cathode and thelayer of liquid aluminum have an edge length in the region of approx. 5cm.

The anodes and cathodes are otherwise modeled with an unrestricted sizeof the grid cells, wherein the size of the grid cells can vary betweenfine regions (e.g. at the melt layer) and coarse regions (e.g. at thesupplies and discharges). The magnification factor between two adjacentgrid cells is limited to at most 1.65 in order to avoid distorted gridelements.

In the case of electrolysis cells defined below and operated underturbulent flow conditions, the solid boundary surfaces between theindividual components of the electrolysis cell are modeled in the cellconstruction by so-called Inflation Boundary Layers available in ComsolMultiphysics, which comprise prismatic cells (in contrast with, forexample, tetrahedral elements).

The individual grid cells of the grid structure thus constructed arethen provided with corresponding material properties, i.e. the gridcells are provided in particular with values for the specific electricalresistance and the grid cells representing the layer of liquid aluminumand the melt layer are additionally provided with values for theviscosity and density of the aluminum and the melt.

The following values are taken as a basis for the material properties:

TABLE 2 Specific resistance [in Ohm · m] Cathode  1.2 · 10⁻⁵ Busbarsmade of steel 7.78 · 10⁻⁷ Liquid aluminum  2.8 · 10⁻⁷ Melt 4.84 · 10⁻³Anode  4.0 · 10⁻⁵ Anode tree (aluminum) 2.34 · 10⁻⁷ Viscosity [in Pa ·s] Liquid aluminum  9.0 · 10⁻⁵ Melt 2.34 · 10⁻³ Density [in kg/m³]Liquid aluminum  2.3 · 10³ Melt 2.08 · 10³

All the other material properties used in the simulation are selectedsuch that they correspond to the actual properties of the respectivematerial.

For the numeric stabilization of the electromagnetic and flow-mechanicalcalculations, the—in reality—abrupt transition of the materialproperties at the boundary surface between the layer of liquid aluminumand the melt layer are also smoothed out in the simulated structure in arange of ±3 cm, i.e. the cells of the grid structure representing thelayer of liquid aluminum and the melt layer which are located within arange of 3 cm below and above the boundary surface are provided withvalues for the material properties which are selected such that, in thisrange, an essentially linear property transition results from theproperties of the cells representing the aluminum layer given in abovetable 2 to the properties of the cells representing the melt layer givenin table 2.

The air surrounding the electrolysis cell is provided with anartificially high specific electrical resistance of 1 Ohm·m, so that itdoes not contribute to the current transport.

For the grid structure thus constructed, which reproduces theelectrolysis cell in its geometry and therewith its material properties,the electromagnetic fields are calculated and the ascertained the valuesare then inserted into the calculation of the flow-mechanical movementsof the melt of the electrolysis cell.

The first step of the modeling of the electromagnetics is based on theknown stationary Maxwell equations:

∇·J=10

∇×H=j

J=σE+J _(e)

E=−∇V

B=∇×A

Lagrange functions (1st order for V and 2nd order for A) are used asstarting functions for the finite element methods.

These partial differential equations are solved for the whole geometryby numeric calculation. The boundary conditions to be used thereby areexplained more precisely below; in particular, the operating current ofthe electrolysis cell fed through the cathode and anode enters into thecalculation as an operating parameter preset from the exterior.

The Lorentz force density thus calculated is then used as a basis forthe calculation of the flow mechanics in the bath of the electrolysiscell.

Depending on the nature of the flow, the flow-mechanical calculation isbased on different equations. In order to select the partialdifferential equations to be used, the known Reynold's number

${Re} = \frac{\rho \; v_{C}L_{C}}{\mu}$

is used and, depending thereon, the following equation systems:

The following equations (Navier-Stokes equations) are used for laminarand weakly turbulent problems with Re<10,000:

${{\rho \left( {u \cdot \nabla} \right)}u} = {{\nabla{\cdot \left\lbrack {{\rho \; I} + {\mu \left( {{\nabla\; u} + \left( {\nabla u} \right)^{T}} \right)} - {\frac{2}{3}{\mu \left( {\nabla{\cdot u}} \right)}I}} \right\rbrack}} + F}$∇⋅(ρ u) = 0

Lagrange functions (1st order for p and 2nd order for u) are used asstarting functions for the finite element methods.

The following equations (Low Reynold's k-epsilon equations) are used forflows in the transition region with Re≧10,000 and <100,000:

${{\rho \left( {u \cdot \nabla} \right)}u} = {{\nabla{\cdot \left\lbrack {{{- \rho}\; I} + {\left( {\mu + \mu_{T}} \right)\left( {{\nabla u} + \left( {\nabla u} \right)^{T}} \right)} - {\frac{2}{3}\left( {\mu + \mu_{T}} \right)\left( {\nabla{\cdot u}} \right)I} - {\frac{2}{3}\rho \; {kI}}} \right\rbrack}} + F}$     ∇⋅(ρ u) = 0$\mspace{79mu} {{{\rho \left( {u \cdot \nabla} \right)}k} = {{\nabla{\cdot \left\lbrack {\left( {\mu + \frac{\mu_{T}}{\sigma_{k}}} \right){\nabla k}} \right\rbrack}} + P_{k} - {\rho ɛ}}}$${{{\rho \left( {u \cdot \nabla} \right)}ɛ} = {{\nabla{\cdot \left\lbrack {\left( {\mu + \frac{\mu_{T}}{\sigma_{e}}} \right){\nabla ɛ}} \right\rbrack}} + {C_{ɛ\; 1}\frac{ɛ}{k}P_{k}} - {C_{ɛ2}\rho \frac{ɛ^{2}}{k}{f_{ɛ}\left( {\rho,\mu,k,ɛ,I_{w}} \right)}}}},\mspace{79mu} {ɛ = {ep}}$$\mspace{79mu} {{{{{\nabla G} \cdot {\nabla G}} + {\sigma_{w}{G\left( {\nabla{\cdot {\nabla G}}} \right)}}} = {\left( {1 + {2\sigma_{w}}} \right)G^{4}}},\mspace{79mu} {I_{w} = {\frac{1}{G} - \frac{I_{ref}}{2}}}}$$\mspace{79mu} {{\mu_{T} = {\rho \; C_{\mu}\frac{k^{2}}{ɛ}{f_{\mu}\left( {\rho,\mu,k,ɛ,I_{w}} \right)}}},\mspace{79mu} {P_{k} = {{\mu_{T}\left\lbrack {\nabla{u:{\left( {{\nabla u} + \left( {\nabla u} \right)^{T}} \right) - {\frac{2}{3}\left( {\nabla{\cdot u}} \right)^{2}}}}} \right\rbrack} - {\frac{2}{3}\rho \; k{\nabla{\cdot u}}}}}}$$\mspace{79mu} {f_{\mu} = {\left( {1 - ^{{- l^{*}}/14}} \right)^{2} \cdot \left( {1 + {\frac{5}{R_{t}^{3/4}}^{- {({R/200})}^{2}}}} \right)}}$     f_(ɛ) = (1 − ^(−l^(*)/3.1))² ⋅ (1 − 0.3^(−(R/6.5)²))     l^(*) = (ρ u_(ɛ)l_(w))/μ      R_(t) = ρ k²/(μɛ)     u_(ɛ) = (μɛ/ρ)^(1/4)

Lagrange functions (1st order for p and 2nd order for u, k and ep) areused as starting functions for the finite element methods.

The following equations (k-epsilon equations) are used for turbulentflows with Re≧100,000:

${{\rho \left( {u \cdot \nabla} \right)}u} = {{\nabla{\cdot \left\lbrack {{{- \rho}\; I} + {\left( {\mu + \mu_{T}} \right)\left( {{\nabla u} + \left( {\nabla u} \right)^{T}} \right)} - {\frac{2}{3}\left( {\mu + \mu_{T}} \right)\left( {\nabla{\cdot u}} \right)I} - {\frac{2}{3}\rho \; {kI}}} \right\rbrack}} + F}$     ∇⋅(ρ u) = 0$\mspace{79mu} {{{\rho \left( {u \cdot \nabla} \right)}k} = {{\nabla{\cdot \left\lbrack {\left( {\mu + \frac{\mu_{T}}{\sigma_{k}}} \right){\nabla k}} \right\rbrack}} + P_{k} - {\rho ɛ}}}$$\mspace{79mu} {{{{\rho \left( {u \cdot \nabla} \right)}ɛ} = {{\nabla{\cdot \left\lbrack {\left( {\mu + \frac{\mu_{T}}{\sigma_{c}}} \right){\nabla ɛ}} \right\rbrack}} + {C_{ɛ\; 1}\frac{ɛ}{k}P_{k}} - {C_{ɛ\; 2}\rho \frac{ɛ^{2}}{k}}}},\mspace{79mu} {ɛ = {ep}}}$$\mspace{79mu} {{\mu_{T} = {\rho \; C_{\mu}\frac{k^{2}}{ɛ}}},\mspace{79mu} {P_{k} = {{\mu_{T}\left\lbrack {\nabla{u:{\left( {{\nabla u} + \left( {\nabla u} \right)^{T}} \right) - {\frac{2}{3}\left( {\nabla{\cdot u}} \right)^{2}}}}} \right\rbrack} - {\frac{2}{3}\rho \; k{\nabla{\cdot u}}}}}}$

wherein C_(μ)0.09; C_(ε1)=1.44; C_(ε2)=1.02; σ_(k)=1.0 and σ_(ε)=1.3.

Lagrange functions (1st order for p and 2nd order for u, k and ep) areused as starting functions for the finite element methods.

The values previously calculated in the electromagnetic consideration inthe form of the Lorentz force density {right arrow over(F)}_(Ltz)={right arrow over (j)}×{right arrow over (B)} also enter intothe above equations. Lorentz force density {right arrow over (F)}_(Liz)forms, together with gravitational force density {right arrow over(F)}_(g)=−ρg{right arrow over (e)}_(z) external excitation F accordingto {right arrow over (F)}={right arrow over (F)}_(Ltz)+{right arrow over(F)}g contained in the above equations.

The above flow-mechanical partial differential equations are also solvednumerically.

In the context of the aforementioned calculations, use is also made ofthe following boundary conditions:

The following boundary conditions relate to the electric fieldscalculated during the electromagnetic calculation:

-   -   The external faces of the treated volume are regarded as an        electrical insulator (−n·j=0).    -   Any symmetrical faces present are regarded as an electrical        insulator (−n·j=0).    -   An electric voltage V is applied at the input of the anode tree,        which is adapted such that the cell current (e.g. 168 kA)        intended for the normal operation of the electrolysis cell        flows.    -   An electric voltage V of 0 volt is applied to the cathode-side        current discharge (earthing).    -   The calculated electrical potential V is continuous at all the        internal faces.

The following boundary conditions relate to the magnetic fieldscalculated during the electromagnetic calculation:

-   -   The magnetic flux is parallel to the external face (n×A=0) at        the external faces of the treated volume.    -   A magnetic symmetry (n×H=0) is present at any symmetrical faces        that may be present.    -   The calculated magnetic vector potential A is continuous at all        the internal faces.

The following boundary conditions relate to the flow fields calculatedduring the flow-mechanical calculation:

-   -   The following holds at the solid boundary surfaces:        -   When use is made of the laminar equations: The liquid            adheres firmly to the solid boundary surface, which is also            denoted as “No slip”, i.e. the speed u=0.        -   When use is made of the turbulent equations, a wall model is            used which takes account of the friction between the            respective liquid layer and the solid boundary surface.    -   An open boundary surface is present at any symmetrical faces        that may be present, wherein the normal flow in relation to the        boundary surface is calculated with f₀=0 according to the        equation

${{\left\lbrack {{{- \rho}\; I} + {\mu \left( {{\nabla u} + \left( {\nabla u} \right)^{T}} \right)} - {\frac{2}{3}{\mu \left( {\nabla{\cdot u}} \right)}I}} \right\rbrack n} = {{- f_{0}}n}},{{u \cdot t} = 0}$

-   -   The calculated flow rate u is continuous at all the internal        faces (e.g. the boundary surface between the layer of liquid        aluminum and the melt layer).

As described above, electromagnetic magnitudes V, A_(x), A_(y), A_(z),j_(x), j_(y a) and j_(z) are first calculated with the aid of theMaxwell equations and the Lorentz force density resulting therefrom isthen inserted into the flow-mechanical equations used in each case inorder to calculate therefrom flow field magnitudes u_(x), u_(y), u_(z a)and p. The electromagnetic calculation and the flow-mechanicalcalculation are therefore coupled together in a unidirectional manner.

Iterative solvers (GMRES) with geometrical multigrid preconditioning areused in each case to solve the partial differential equations statedabove. If need be, use is made of standard stabilisation techniques forflow mechanics such as the Streamline Diffusion (GLS) available inComsol Multiphysics and Crosswind Diffusion as well as calibration ofthe vector potential in the electromagnetic calculation.

According to the invention, the surface profiling of the cathodeaccording to the invention comprises two or more elevations, wherein anelevation is provided in each case at at least two of the twenty pointsof the surface of the upper side of the cathode which in each case aredisposed vertically beneath those regions of the boundary surfacebetween the layer of liquid aluminum and the melt layer in which thepeaks with the twenty highest maximum values are present in thedistribution of the reference wave formation potential present in theboundary surface. In a development of the inventive idea, it is proposedthat an elevation is provided in each case at at least X of the Y pointsof the surface of the upper side of the cathode which in each case aredisposed vertically beneath those regions of the boundary surfacebetween the layer of liquid aluminum and the melt layer in which thepeaks with the Y highest maximum values are present in the distributionof the reference wave formation potential present in the boundarysurface,

wherein X=4 and Y=20, preferably X=6 and Y=20, particularly preferablyX=10 and Y=20 and very particularly preferably X=14 and Y=20 and/or

wherein X=2 and Y=10, preferably X=3 and Y=10, particularly preferablyX=5 and Y=10 and very particularly preferably X=7 and Y=10 and/or

wherein X=1 and Y=5, preferably X=2 and Y=5, particularly preferably X=3and Y=5 and very particularly preferably X=4 and Y=5. In this way,marked peaks in the wave formation potential of the electrolysis cellare avoided particularly comprehensively, so that the stability of theelectrolysis cell during operation is increased still further.

According to a further preferred embodiment of the invention, provisionis made such that at least one of the elevations disposed at the pointsof the surface of the upper side of the cathode which in each case aredisposed vertically beneath those regions of the boundary surfacebetween the layer of liquid aluminum and the melt layer in which in eachcase a peak is present in the distribution of the reference waveformation potential present in the boundary surface has its maximumheight at the point disposed vertically beneath the point of theboundary surface between the layer of liquid aluminum and the melt layerat which the peak of the distribution of the reference wave formationpotential has its maximum value. An excessive wave formation in thecorresponding region of the boundary surface is thus particularlyeffectively avoided.

It is particularly preferable for the essentially congruent arrangementdescribed above to be guaranteed for all peak-elevation pairs, i.e. allthe elevations at the points of the surface of the upper side of thecathode which in each case are disposed vertically beneath those regionsof the boundary surface between the layer of liquid aluminum and themelt layer in which a peak is in each case present in the distributionof the reference wave formation potential present in the boundarysurface have in each case their maximum height at the point disposedvertically beneath the point of the boundary surface between the layerof liquid aluminum and the melt layer at which the respective peaks ofthe distribution of the reference wave formation potential have theirmaximum value.

A particularly effective compensation of a peak in the distribution ofthe reference wave formation potential is achieved if the geometricalouter contour of at least one of the elevations is at least essentiallysimilar in plan view to the geometrical outer contour of the respectivepeak of the distribution of the reference wave formation potential inplan view or essentially corresponds to the latter.

Similarity is understood in agreement with the commonly used specialistlinguistic usage, that the two outer contours can be transferred intoone another by geometrical mapping, which can be composed of concentricelongations and congruence mappings, such as in particulardisplacements, rotations or mirroring. For example, the two outercontours can essentially correspond to a circle. The two outer contourscan also form essentially two triangles which have two essentiallyidentical angles, or form essentially two rectangles with at leastapproximately equal side ratios or form essentially two ellipses with atleast approximately identical numeric eccentricities.

It is particularly preferable for the geometrical outer contours of allthe elevations in plan view to be at least essentially similar to thegeometrical outer contour of the respective peaks of the distribution ofthe reference wave formation potential in plan view or essentiallycorrespond to the latter.

According to a further preferred embodiment of the present invention,the geometrical outer contour of at least one of the elevations in planview is constituted at least in sections at least approximatelypolygonal and/or ellipsoidal. Such elevations can be producedparticularly easily and are particularly well suited for effectivelycompensating for a peak of the reference wave formation potential. Anelevation that is particularly easy to produce emerges when theelevation constituted as a polygon has 3, 4, 5 or 6 corners.

Within the scope of the present invention, the outer contour of anelevation viewed vertically from above can advantageously be selectedsuch that it can be generated by a simplification of the outer contourof the respective peak of the distribution of the reference waveformation potential viewed vertically from above. At least one of theelevations thus preferably has an outer contour, viewed in plan view,which is geometrically simpler than the outer contour, viewed in planview, of the peak of the distribution of the reference wave formationpotential disposed vertically above the elevation in the boundarysurface. It is preferable for the sum of the numbers of all the cornersand all the differently curved regions of the outer contour area of theelevation viewed from above to be smaller than the sum of all thecorners and all the differently curved regions of the outer contourarea, viewed from above, of the corresponding peak of the distributionof the reference wave formation potential. All the sections of the outercontour following one another in the peripheral direction, between whicha point of inflection is located, are counted as differently curvedregions of an outer contour.

In order to prevent particularly effectively an increased wave formationof the layer of liquid aluminum caused by a peak in the reference waveformation potential, it has proved to be advantageous if thethree-dimensional shape of at least one of the elevations is at leastessentially similar to the three-dimensional shape of the respectivepeak of the distribution of the reference wave formation potential oressentially corresponds to the latter.

It is particularly preferred if the three-dimensional shapes of all theelevations are at least essentially similar to the three-dimensionalshape of the respective peak of the distribution of the reference waveformation potential or essentially correspond to the latter.

According to a further advantageous embodiment of the invention, atleast one of the elevations has a three-dimensional shape taperingupwards in the vertical direction. This formation leads to aparticularly effective avoidance of wave formation in the region of apeak in the distribution of the reference wave formation potential. Theat least one elevation, when viewed from the side, can for example havean essentially polygonal and preferably essentially trapezoidal outercontour.

In a development of the inventive idea, it is proposed that at least oneof the elevations, viewed upwards in the vertical direction, is boundedby a top surface which, viewed in plan view, has a smaller area than thebase surface of the elevation viewed in plan view. The elevation can forexample be constituted at least approximately conical or in the shape ofa truncated pyramid.

According to a further embodiment of the invention, at least one of theelevations has a three-dimensional shape which, proceeding from the basesurface of the elevation, can be generated by rotating the base surfaceabout an axis of rotation bordering the base surface. The axis ofrotation preferably runs essentially horizontally. Such elevationgeometries are particularly well suited for effectively making thedistribution of the wave formation potential uniform and are alsoparticularly easy to produce. The at least one elevation can preferablybe generated by rotating the base surface around the axis of rotationthrough an angle of at least 75° and at most 180°.

A further advantageous embodiment of the present invention ischaracterized in that at least one of the elevations has athree-dimensional shape which, proceeding from the base surface of theelevation, can be generated by geometrical extrusion of the base surfaceof the elevation upwards in the vertical direction. The extrusiondirection is preferably at least approximately vertical and diverges upto 45° from the vertical direction. The elevation is preferably reducedin size in the extrusion direction by scaling in the course of theextrusion. In principle, it is preferable during the extrusion for theat least one elevation to taper upwards in the vertical direction. Theintroduction of elevations is also possible by means of vacuumvibration, uniaxial pressing or another suitable forming process.

In the case of the electrolysis cell, the cathode can comprise two ormore cathode blocks and/or the anode can comprise two or more anodeblocks. In particular, a plurality of cathode blocks can be disposed insuccession beside one another viewed in the transverse direction of thecathode blocks and can be connected along their longitudinal sides bymeans of a tamping compound. Furthermore, it is preferable that, viewedin the width direction of the cathode blocks, an anode block covers twocathode blocks and, viewed in the longitudinal direction of the cathodeblocks, two anode blocks cover a cathode block.

A particularly high energy efficiency of the electrolysis cell can beachieved if the distance between the anode and the layer of liquidaluminum amounts to between 15 and 45 mm, preferably between 15 and 35mm and particularly preferably between 15 and 25 mm. This small distanceis achieved by the reduction in the wave formation potentials and bymaking the distribution of the wave formation potential uniform.

As described above, the surface profiling of the cathode is adaptedaccording to the invention in such a way that marked peaks of the waveformation potential at individual points of the boundary surface betweenthe layer of liquid aluminum and the melt layer are avoided. The resultis surface profilings which are adapted in their position, size andshape to the specific properties of the electrolysis cell determiningthe wave formation potential. The present invention abandons, in adeliberate and targeted manner, the route of defining a-priori a surfaceprofiling which is constituted regular, but which by the same token isnot adapted to the wave formation potential present in each case.Instead, the surface profiling of the cathode of an electrolysis cellaccording to the invention is constituted in practice irregular at leastin one direction.

The present invention further relates to a cathode for an aluminumelectrolysis cell, the upper side whereof comprises a surface profilingwith two or more first webs running essentially in a first direction ofthe cathode and at least one second web running at least essentially inthe direction normal to the first direction of the cathode.

Within the meaning of the present invention, a web is regarded as anelevation running at least essentially straight in the longitudinaldirection.

Within the scope of the present invention, it has been shown that acathode with such a surface profiling is suitable for achieving, whenused in electrolysis cells, a distribution of the wave formationpotential in the boundary surface between the layer of liquid aluminumand the melt layer during operation of the given electrolysis cell,wherein marked peaks of the wave formation potential at individualpoints of the boundary surface are effectively avoided. The specificallydescribed surface profiling is adapted to the conditions that prevail ina large number of commonly available electrolysis cells, and isconstituted such as to achieve uniform distribution of the waveformation potential in these electrolysis cells taking account of theseconditions.

Such a cathode can in particular be a component part of one of thepreviously described electrolysis cells according to the invention.

In addition, the cathode according to the invention is preeminentlywell-suited, when used in electrolysis cells, for achieving theadvantages of an improved energy efficiency and an increased servicelife and at the same time ensuring sufficient mixing of the melt in theelectrolysis cell.

According to an advantageous embodiment of the present invention, the atleast two first webs run at least approximately in a transversedirection of the cathode.

In a development of the inventive idea, it is proposed that the upperside of the cathode has, in plan view, an essentially rectangular outercontour, wherein an elevation of the cathode is provided at least in oneof the four corners of the essentially rectangular outer contour. Withinthe scope of the present invention, it has been shown that marked peaksin the distribution of the reference wave formation potential areusually present in these corner regions, so that the stability of theelectrolysis cell during operation can be considerably increased by themeasure according to the invention. The elevation disposed in the cornerregion preferably has, in plan view, an essentially triangular outercontour.

A further advantageous embodiment of the invention makes provision suchthat the upper side of the cathode comprises a depression in the form ofa trough which is at least essentially V-shaped when viewed in thecross-section of the cathode. The depression constituted in the form ofa V-shaped trough serves to reduce the current density in the lateraledge regions of the cathode, which is otherwise increased on account ofthe contact taking place there with the busbars inserted in the cathodebase, and thus to reduce the wave formation potential in these regions.

The at least two first webs and that the at least one second web arepreferably disposed on the surface of the essentially V-shapeddepression.

According to a further advantageous embodiment of the present invention,the connection point between the two legs of the cross-section of thedepression constituted essentially in the form of a V-shaped trough, asviewed in the cross-section of the cathode, is disposed at leastapproximately in the middle of the cathode. In this way, the electriccurrent density is displaced from the lateral edge regions of thecathode cross-section into the middle, in order to reduce peaks in thecurrent density in these edge regions when the cathode is used in anelectrolysis cell and to achieve a low wave formation potential and anessentially uniform distribution of the wave formation potential.

In the present invention, it has proved to be advantageous for thedepression to extend over at least 75%, preferably over at least 90%,particularly preferably over at least 95% and very particularlypreferably over 100% of the surface of the cathode. In this way, thedistribution of the wave formation potential is made uniform over thewhole cathode surface when the cathode is used in an electrolysis cell.

The at least one second web, when viewed in the second direction of thecathode, is preferably disposed at least approximately in the middle ofthe cathode. Since an excessively high wave formation potential isotherwise to be expected in this region, a particularly favorable effecton the wave formation potential is thus achieved.

According to a further advantageous embodiment, the upper edge of atleast one of the first webs has a distance from the bottom of theV-shaped trough increasing towards the middle of the cathode, as viewedin the transverse direction of the cathode. This increase in thedistance towards the middle of the cathode block serves to avoidexcessive peaks in the wave formation potential in the middle of thecathode block and thus an increased wave formation in the layer ofliquid aluminum in this region when the cathode block is used in anelectrolysis cell.

A further subject-matter of the present invention is an electrolysiscell, in particular for the production of aluminum, which comprises atleast one cathode as described above, on the upper side of the cathode alayer of liquid aluminum, thereon a melt layer and above the melt layeran anode. The advantages and embodiments described above with regard tothe cathode also apply accordingly to the electrolysis cell according tothe invention.

According to an advantageous embodiment of the present invention, theanode comprises at least two anode blocks disposed beside one another,wherein a joint extends between the at least two anode blocks andwherein at least one of the first webs of the cathode is disposedvertically beneath and at least essentially parallel to the jointconstituted between the two anode blocks. The angular divergence betweenthe orientation of the webs and the orientation of the joint preferablyamounts to a maximum of 20°. According to the invention, it has beenrecognised that these regions between the anode blocks usually have amarkedly increased wave formation potential, so that the describedmeasure contributes towards increasing the stability of the electrolysiscell still further. The at least one first web disposed verticallybeneath the joint is preferably disposed in an at least approximatelycentered manner in relation to the joint.

A further subject-matter of the present invention is a method for theproduction of an electrolysis cell.

The method for the production of an electrolysis cell, in particular anelectrolysis cell for the production of aluminum, which comprises acathode, on the upper side of the cathode a layer of liquid aluminum,thereon a melt layer and above the melt layer an anode, comprises thefollowing steps: ascertaining the distribution of the reference waveformation potential present in the boundary surface between the layer ofliquid aluminum and the melt layer of the electrolysis cell;

producing a surface profiling comprising a plurality of elevations onthe upper side of the cathode, wherein an elevation is provided in eachcase at at least two of the twenty points of the surface of the upperside of the cathode which in each case are disposed vertically beneaththose regions of the boundary surface between the layer of liquidaluminum and the melt layer in which the peaks with the twenty highestmaximum values are present in the distribution of the reference waveformation potential present in the boundary surface, wherein a referencewave formation potential is defined as the wave formation potentialwhich, during the operation of the electrolysis cell with—instead of thecathode with the surface profiling—a reference cathode without surfaceprofiling, but an otherwise identical configuration to the cathode withsurface profiling, is present at a point in the boundary surface betweenthe layer of liquid aluminum and the melt layer.

With the method according to the invention, electrolysis cells accordingto the invention as described above can be produced. The advantages andembodiments described above in respect of the electrolysis cellaccording to the invention are accordingly applicable to the methodaccording to the invention.

According to a further claim, there is defined a method for theproduction of an electrolysis cell, in particular an electrolysis cellfor the production of aluminum, which comprises a cathode, on the upperside of the cathode a layer of liquid aluminum, thereon a melt layer andabove the melt layer an anode. The further method comprises the steps:

ascertainment of the distribution of the reference wave formationpotential present in the boundary surface between the layer of liquidaluminum and the melt layer of the electrolysis cell,

production of a surface profiling comprising a plurality of elevationson the upper side of the cathode, wherein an elevation is provided ineach case at at least two of the twenty points of the surface of theupper side of the cathode which in each case are disposed verticallybeneath those regions of the boundary surface between the layer ofliquid aluminum and the melt layer in which the peaks with the twentyhighest maximum values are present in the distribution of the referencewave formation potential present in the boundary surface, wherein areference wave formation potential is defined as the wave formationpotential which, during the operation of the electrolysis cellwith—instead of the cathode with the surface profiling—a referencecathode without surface profiling, but an otherwise identicalconfiguration to the cathode with surface profiling, wherein thereference cathode is disposed with regard to its height in theelectrolysis cell in such a way that the same volume for the layer ofliquid aluminum and the melt layer is provided between the referenceelectrode and the anode as in the case of the electrolysis cell with thecathode with surface profiling, is present at a point in the boundarysurface between the layer of liquid aluminum and the melt layer.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin an electrolysis cell and cathode with irregular surface profiling, itis nevertheless not intended to be limited to the details shown, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 shows an electrolysis cell according to an embodiment of theinvention in perspective view;

FIG. 2 shows the local distribution of the reference wave formationpotential of the electrolysis cell of FIG. 1 in the boundary surfacebetween the layer of liquid aluminum and the melt layer in plan view;

FIG. 3 shows the surface-profiled cathode of the electrolysis cell ofFIG. 1 in plan view;

FIG. 4 shows the cathode of FIG. 3 in perspective view;

FIG. 5 shows the local distribution of the wave formation potential inthe boundary surface between the layer of liquid aluminum and the meltlayer of the electrolysis cell of FIGS. 1 to 4;

FIG. 6 (in partial views 6A to 6L) shows exemplary elevations for asurface profiling according to the invention; and

FIG. 7 (in partial views 7A to 7L) shows further exemplary elevationsfor a surface profiling according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is shown an electrolysis cell 10for the production of aluminum comprising a cathode 12, on the upperside of cathode 10 a layer 14 of liquid aluminum, thereon a melt layer16 and above melt layer 16 an anode 18. Layer 14 of liquid aluminum andmelt layer 16 merge into one another at a boundary surface 15.

Cathode 12 comprises a plurality of elongated cathode blocks whichextend in transverse direction y of electrolysis cell 10 and which aredisposed beside one another in longitudinal direction x of electrolysiscell 10 and are connected to one another via a tamping-compound jointnot represented. A busbar 20, which extends through the cathode block inlongitudinal direction y of the cathode block and which makes electricalcontact with the cathode block, is inserted at the underside of eachcathode block.

Busbars 20 are grouped together electrically via a current discharge 22,which is constituted geometrically in such a way that a magnetic fieldcompensation is brought about, i.e. that the distribution of magneticflux density B brought about by the current flow is to a certain degreemade uniform.

Anode 18 comprises a multiplicity of anode blocks 24 which are connectedto one another via a current supply 28 comprising an anode tree 26.

Cathode 12 of electrolysis cell 10 comprises a surface profilingcomprising a plurality of elevations 30, said surface profiling beingadapted, as explained below, to the distribution of the reference waveformation potential of electrolysis cell 10 in boundary surface 15.

In the present example of embodiment, the fact that electrolysis cell 10shown in FIG. 1 is mirror-symmetrical with respect to plane of symmetry32 can be used for the calculation of the reference wave formationpotential. In the calculation of the reference wave formation potential,therefore, only the half of the electrolysis cell located on one side ofplane of symmetry 32 of the electrolytic cell must be explicitlyincluded in the simulated volume, wherein the symmetry is taken intoaccount by corresponding boundary conditions at the edge of thesimulation volume corresponding to plane of symmetry 32.

FIG. 2 shows the distribution of the reference wave formation potentialpresent at boundary surface 15 of electrolysis cell 10 from FIG. 1, asviewed from above, for one of the two symmetrical halves of electrolysiscell 10, wherein equipotential lines of the reference wave formationpotential are shown specifically in FIG. 2. The outer contour of cathode12 of electrolysis cell 10 is also represented.

As can be seen from FIG. 2, the reference wave formation potential ofelectrolysis cell 10 comprises a plurality of peaks 34, the maximumheights whereof can be seen in FIG. 2 on the basis of the number ofclosed equipotential lines lying inside one another.

FIG. 3 shows one of the symmetrical halves of cathode 12 of electrolysiscell 10 from FIG. 1 in plan view. As a comparison of FIG. 3 and FIG. 2shows, elevations 30 of the surface profiling of cathode 12 are disposedin each case vertically beneath peaks 34 of the reference wave formationpotential, wherein peaks 34 and elevations 30, when viewed from above,are disposed essentially congruent above one another. The correspondencebetween peaks 34 in FIG. 2 and elevations 30 in FIG. 3 is marked by thecorresponding letter endings of reference numbers 30 and 34, i.e.elevation 30 a corresponds to peak 34 a etc.

The shape of elevations 30 is adapted to the shape of respectivelyassigned peaks 34 of the reference wave formation potential, whereinelevations 30 approximate to the shape of respectively assigned peaks 34in each case by geometrically simplified shapes, such as for example bytwo essentially oval elevations 34 g and 34 j with ellipsoidal outercontours, an elevation 30 h in the shape of a truncated pyramid, aplurality of elevations 34 b, c, e, l, m, n, o in the shape of atruncated half-pyramid and two elevations 34 a and 34 c in the shape ofa truncated quarter-pyramid in the corner regions of cathode 12.

FIG. 4 illustrates in a perspective representation the three-dimensionalshape of elevations 30 adapted to the reference wave formationpotential. Grooves 37, for busbars 20, disposed at the underside ofcathode 12 can also be seen here (FIG. 1).

The distribution of the wave formation potential of electrolysis cell 10with surface-profiled cathode 16 present in boundary surface 15 betweenlayer 14 of liquid aluminum and melt layer 16 is shown in FIG. 5. As acomparison of the distribution of the wave formation potential shown inFIG. 5 with the distribution of the reference wave formation potentialshown in FIG. 2 shows, the creation of a considerable uniformity orsmoothing and a reduction in the height of peaks 34 in the distributionof the wave formation potential is achieved by means of the surfaceprofiling. Specifically, FIG. 5 shows only peaks 34 which exhibit amaximum of two closed equipotential lines lying within one another.Thus, the maximum wave formation potential in the corresponding regionsof boundary surface 15 is much smaller than in the case of thedistribution of the reference wave formation potential. The stability ofelectrolysis cell 10 during operation is thus considerably increased anda longer service life and higher energy efficiency of electrolysis cell10 are thus achieved.

FIG. 6 and FIG. 7 show exemplary elevations 30 which are particularlysuitable for a surface profiling of an electrolysis cell 10 according tothe invention. Elevations 30 shown in FIG. 6 can in each case begenerated by geometrical extrusion. FIG. 6 a-c show respectivelypolygonal, ellipsoidal and other base surfaces 36, proceeding from whichelevation 30 is extruded. As indicated by an arrow 39, the extrusiontakes place in each case in vertical direction z.

FIG. 6 d shows an elevation 30 in the shape of a truncated pyramid,extruded proceeding from the base surface of FIG. 6 a. The geometricalextrusion in vertical direction z comprises a scaling of the area withincreasing vertical height, so that resultant elevation 30 iscontinuously tapered upwards. The reference axis of the scaling is thevertical axis proceeding from centroid point 38 of base surface 36.Elevation 30 shown in FIG. 6 d results from an isotropic scaling,wherein the area is contracted in all directions normal to the extrusiondirection, as it were to the extrusion axis. FIG. 6 e also shows anelevation 30 extruded from base surface 36 of FIG. 6 a, in which howeveran anisotropic scaling takes place, i.e. the area is scaled verydifferently in different directions normal to the extrusion direction.Elevation 30 of FIG. 6 e corresponds, moreover, to an elevation whichhas been extruded along an axis diverging from the vertical by a smallangular amount. Centroid point 38 of top surface 40 of resultantelevation 30 in FIG. 6 e, in contrast with centroid point 28 of topsurface 40 in FIG. 6 d, is accordingly horizontally displaced withrespect to centroid point 38 of base surface 36.

FIG. 6 f and FIG. 6 g show in each case an elevation 30 extrudedproceeding from ellipsoidal base surface 36 shown in FIG. 6 b, whereinelevation 30 shown in FIG. 6 f results from an isotropic and theelevation shown in FIG. 6 g from an anisotropic scaling of the area inthe extrusion direction.

FIG. 6 h and FIG. 6 i show in each case an elevation 30 extrudedproceeding from base surface 36 shown in FIG. 6 c, wherein elevation 30shown in FIG. 6 h results from an isotropic and the elevation shown inFIG. 6 i from an anisotropic scaling of the area in the extrusiondirection.

FIG. 7 a-i show further exemplary elevations 30, which can be generatedby geometrical rotation of a base surface 36. FIG. 7 a-c show in eachcase different base surfaces 36, i.e. a polygonal base surface 30 inFIG. 7 a, a semi-ellipsoidal base surface 30 in FIG. 7 b and a freelyselected base surface 30 in FIG. 7 c. An edge line of base surfaces 36in each case forms axis of rotation 42 for the rotation.

FIG. 7 d and FIG. 7 e show in each case elevations 30, which aregenerated proceeding from polygonal base surface 36 in FIG. 7 a,wherein, according to FIG. 7 d, the rotation body is generatedexclusively by rotation and, according to FIG. 7 e, the resultantrotation body is scaled anisotropically once again with respect to basesurface 36 and the direction lying normal thereto.

FIG. 7 f and FIG. 7 g show in each case elevations 30, which aregenerated proceeding from semi-ellipsoidal base surface 36 in FIG. 7 b,wherein, in FIG. 7 f, the rotation body is generated exclusively byrotation and, in FIG. 7 g, the resultant rotation body is scaledanisotropically once again with respect to base surface 36 and thedirection lying normal thereto.

FIG. 7 h and FIG. 7 i show in each case elevations 30, which aregenerated proceeding from base surface 36 in FIG. 7 c, wherein, in FIG.7 h, the rotation body is generated exclusively by rotation and, in FIG.7 i, the resultant rotation body is scaled anisotropically once againwith respect to base surface 36 and the direction lying normal thereto.

1. An electrolysis cell for the production of aluminum, comprising: acathode having an upper side formed with a surface profiling of two ormore elevations; during an operation of the electrolysis cell, a layerof liquid aluminum on said upper side of said cathode, a melt layer onthe layer of liquid aluminum, and an anode above the melt layer; saidsurface profiling of said cathode being configured and disposed in sucha way that an elevation is in each case provided at two or more oftwenty points of a surface of said upper side of said cathode that arein each case disposed vertically beneath those regions of a boundarysurface between the layer of liquid aluminum and the melt layer in whichpeaks with the twenty highest maximum values are present in adistribution of a reference wave formation potential present in theboundary surface; the reference wave formation potential being definedas a wave formation potential which, during the operation of theelectrolysis cell with a reference cathode having no surface profilingbeing used instead of said cathode with said surface profiling, buthaving an otherwise identical configuration as said cathode with saidsurface profiling, is present at a point in the boundary surface betweenthe layer of liquid aluminum and the melt layer.
 2. The electrolysiscell according to claim 1, wherein a respective elevation is provided ineach case at at least X of Y points of the surface of said upper side ofsaid cathode which are in each case disposed vertically beneath thoseregions of the boundary surface between the layer of liquid aluminum andthe melt layer in which the peaks with Y highest maximum values arepresent in the distribution of the reference wave formation potentialpresent in the boundary surface, and wherein at least one of thefollowing is true: Y=20 and X is selected from the group consisting of4, 6, 10 and 14; Y=10 and X is selected from the group consisting of 2,3, 5 and 7; Y=5 and X is selected from the group consisting of 1, 2, 3and
 4. 3. The electrolysis cell according to claim 1, wherein at leastone of said elevations disposed at the points of the surface of theupper side of the cathode which in each case are disposed verticallybeneath those regions of the boundary surface between the layer ofliquid aluminum and the melt layer in which a respective peak is presentin the distribution of the reference wave formation potential present inthe boundary surface has its maximum height at the point disposedvertically beneath the point of the boundary surface between the layerof liquid aluminum and the melt layer at which the peak of thedistribution of the reference wave formation potential has its maximumvalue.
 4. The electrolysis cell according to claim 1, wherein ageometrical outer contour of at least one of said elevations issubstantially similar, in plan view, to a geometrical outer contour ofthe respective peaks of the distribution of the reference wave formationpotential in plan view.
 5. The electrolysis cell according to claim 1,wherein a geometrical outer contour of at least one of said elevations,in plan view, at least partially represents a substantial polygon with3, 4, 5 or 6 corners or a substantial ellipsoid.
 6. The electrolysiscell according to claim 1, wherein a three-dimensional shape of at leastone of said elevations is substantially similar to a three-dimensionalshape of the respective peak of the distribution of the reference waveformation potential or the three-dimensional shape of the respectivesaid elevation corresponds to the three-dimensional shape of therespective said peak.
 7. The electrolysis cell according to claim 1,wherein at least one of said elevations is upwardly tapered in verticaldirection.
 8. The electrolysis cell according to claim 1, wherein saidsurface profiling of said cathode is irregularly configured in at leastone direction.
 9. An electrolysis cell for the production of aluminum,comprising: a cathode having an upper side formed with a surfaceprofiling of two or more elevations; during an operation of theelectrolysis cell, a layer of liquid aluminum on said upper side of saidcathode, a melt layer on the layer of liquid aluminum, and an anodeabove the melt layer; said surface profiling of said cathode beingconfigured and disposed in such a way that an elevation is in each caseprovided at two or more of twenty points of a surface of said upper sideof said cathode that are in each case disposed vertically beneath thoseregions of a boundary surface between the layer of liquid aluminum andthe melt layer in which peaks with the twenty highest maximum values arepresent in a distribution of a reference wave formation potentialpresent in the boundary surface; the reference wave formation potentialbeing defined as a wave formation potential which, during the operationof the electrolysis cell with a reference cathode having no surfaceprofiling being used instead of said cathode with said surfaceprofiling, but having an otherwise identical configuration as saidcathode with said surface profiling, and with the reference cathodebeing disposed at a level in the electrolysis cell to provide a volumefor the layer of liquid aluminum and the melt layer between saidreference cathode and said anode equal to a volume provided in theelectrolysis cell with said cathode with said surface profiling, ispresent at a point in the boundary surface between the layer of liquidaluminum and the melt layer.
 10. The electrolysis cell according toclaim 9, wherein a respective elevation is provided in each case at atleast X of Y points of the surface of said upper side of said cathodewhich are in each case disposed vertically beneath those regions of theboundary surface between the layer of liquid aluminum and the melt layerin which the peaks with Y highest maximum values are present in thedistribution of the reference wave formation potential present in theboundary surface, and wherein at least one of the following is true:Y=20 and X is selected from the group consisting of 4, 6, 10 and 14;Y=10 and X is selected from the group consisting of 2, 3, 5 and 7; Y=5and X is selected from the group consisting of 1, 2, 3 and
 4. 11. Theelectrolysis cell according to claim 9, wherein at least one of saidelevations disposed at the points of the surface of the upper side ofthe cathode which in each case are disposed vertically beneath thoseregions of the boundary surface between the layer of liquid aluminum andthe melt layer in which a respective peak is present in the distributionof the reference wave formation potential present in the boundarysurface has its maximum height at the point disposed vertically beneaththe point of the boundary surface between the layer of liquid aluminumand the melt layer at which the peak of the distribution of thereference wave formation potential has its maximum value.
 12. Theelectrolysis cell according to claim 9, wherein a geometrical outercontour of at least one of said elevations is substantially similar, inplan view, to a geometrical outer contour of the respective peaks of thedistribution of the reference wave formation potential in plan view. 13.The electrolysis cell according to claim 9, wherein a geometrical outercontour of at least one of said elevations, in plan view, at leastpartially represents a substantial polygon with 3, 4, 5 or 6 corners ora substantial ellipsoid.
 14. The electrolysis cell according to claim 9,wherein a three-dimensional shape of at least one of said elevations issubstantially similar to a three-dimensional shape of the respectivepeak of the distribution of the reference wave formation potential orthe three-dimensional shape of the respective said elevation correspondsto the three-dimensional shape of the respective said peak.
 15. Theelectrolysis cell according to claim 9, wherein at least one of saidelevations is upwardly tapered in vertical direction.
 16. Theelectrolysis cell according to claim 9, wherein said surface profilingof said cathode is irregularly configured in at least one direction. 17.A cathode for an aluminum electrolysis cell, the cathode comprising: acathode block having an upper side formed with a surface profiling on asurface of the upper side; said surface profiling having a plurality oftwo or more first webs running substantially in a first direction of thecathode and at least one second web running substantially in a seconddirection normal to said first direction of the cathode.
 18. The cathodeaccording to claim 17, wherein said upper side of the cathode is formedwith a depression in the form of a trough having a substantial V-shapewhen viewed in cross-section of the cathode, and wherein said first websand said at least one second web are disposed on a surface of saidsubstantially V-shaped depression.
 19. The cathode according to claim18, wherein said depression extends over at least 75% of a surface ofsaid cathode block.
 20. The cathode according to claim 18, wherein saiddepression extends substantially an entire surface of said cathodeblock.